1. Field of the Invention
The present invention relates generally to imaging systems and more particularly to imaging systems for use in nuclear medicine.
2. Description of the Relevant Art
Gamma ray cameras are used in nuclear medicine to generate high quality images for brain, SPECT (Single Photon Emission Computer Tomograph), and total body bone studies. These cameras are most frequently used for cardiac and total body bone studies.
It is very important that the gamma ray camera be designed for high patient throughput for both economic and therapeutic reasons. The cost for diagnosing each patient is reduced if more patients can be diagnosed per unit time. For very sick patients or patients in intensive care it is important to minimize the time required to acquire image data. Patient throughput is increased if imaging time is reduced. Other factors, such a patient set-up time also affect patient throughput.
Modern gamma ray cameras utilize detectors, such as Anger cameras, having a wide field of view so that it is possible to image the full width of the body of a patient at each angular stop without the requirement of rectilinear scanning. These detectors use thick lead collimators to focus images and are thus very heavy. The collimators must be positioned as close to the patient as possible to acquire image data required to generate high resolution images. The image data acquired by the detectors is processed by a computer to generate an image. Techniques for processing image data are well-know in the art and described in "Principles of Instrumentation in SPECT" by Robert Eisner. Journal of Nuclear Medicine, Vol. 13, #1, March 1985, pp. 23-31; "Computed Tomography in Nuclear Medicine" by John Keyes, (chapter in) Computer Methods, C. V. Mosley, St. Louis, 1977, pp. 130-138; and "Single Photon Emission Computed Tomography," by Bernard Oppenheim and Robert Appledown, (chapter in) Effective Use of Computers in Nuclear Medicine, Michael Gelfand and Stephen Thomas, McGraw-Hill Book Co., New York 1988, pp. 31-74.
Recent technological innovations have produced dual-head systems, with two detectors having their detector image direction arrows oriented at a fixed angle of 180.degree., and triple-head systems, with three detectors having their image direction arrows oriented at fixed angles of 120.degree., SPECT gamma ray cameras capable of rapid, high quality SPECT imaging. FIGS. 1A and 1B are schematic diagrams depicting the fixed orientation of the detector image direction arrows 2 of the detectors 4 in a dual-head and triple-head system.
When the detectors rotate about the patient, a lateral axis is defined as the mechanical axis of rotation aligned with the computer matrix for reconstructing the SPECT images.
The existing, fixed-angle single, dual, and triple head cameras each have certain features which are advantageous for a particular type of application. To determine which system is best for a particular application factors such as 1) the ability of the camera to perform required imaging tasks; 2) the quality of the images generated; and 3) patient throughput are evaluated.
The acquisition of data for a total body scan requires movement of the detector along the length of the patient's body. The dual-head system is very efficient because image data for anterior/posterior images can be acquired simultaneously. The time required to complete a scan can be reduced from 45 to 60 minutes, for a single-head camera, to 30 minutes. The triple-head system is no more efficient than the single-head system because the detectors cannot be aligned to acquire simultaneous anterior/posterior or left/right lateral data.
To generate high-quality SPECT for brain, bone, or liver studies views taken along a complete 360.degree. circle (360.degree. scan) around the body of the patient are required. Typically, about 64 to 128 angular stops are required to acquire the image data. The above-described dual-head camera reduces the imaging time to 1/2 the imaging time of a single-head system because data is acquired from two stops simultaneously. The triple-head camera reduces the imaging time to about 1/3 the imaging time of a single-head system because data is acquired from three stops simultaneously.
For cardiac SPECT studies, the analysis of complex imaging considerations has led to the use of at least 32 stops over a 180.degree. arc about the patient's body (180.degree. scan). For a 180.degree. scan the imaging time of a single-head and dual-head system are the same because only one detector of the dual-head system is within the 180.degree. arc at any given time. A triple-head system reduces the image time so that it is about 2/3 the time of a single-head system for a 180.degree. scan because two detectors are within the 180.degree. arc about 1/3 of the time.
In view of the above it is apparent that the mechanical system for orienting the detectors must be designed to provide a mechanism for accurately orienting the detectors at various angular stops relative to the patient and to position the collimator as close to the patient as possible. Additionally, the system must be stable so that the heavy detectors are held still at each stop to facilitate the acquisition of accurate imaging data. Other attributes that are required of the mechanical system are ease of patient positioning, size of footprint, and overall size of the system.
Further, as described above, the existing systems each have advantages for particular applications but generally lack the flexibility for optimal performance over a range of applications. Additionally, although cardiac SPECT imaging accounts for about 33% of the use of gamma ray cameras, none of the systems significantly reduce the imaging time for the 180.degree. scan used in forming cardiac SPECT images.